Referring to FIG. 1, a conventional motor driving assembly is shown, which drives a fan to rotate. The motor driving assembly includes a driving shaft 71 and a fixing base 73 fixed on the driving shaft 71 at a motor 50 side, and a connecting base 75 fixedly connected to a fan impeller at a fan 60 side. The motor driving assembly further includes a torsional spring 79 connecting the fixing base 73 and the connecting base 75. The connecting base 75 is attached around the driving shaft 71, and the torsional spring 79 is attached around the connecting base 75. Upon startup of the motor 50, the torsional spring 79 is tightened around the connecting base 75, such that the connecting base 75 and the driving shaft 71 generate therebetween a sufficient large frictional force which drives the fan impeller to rotate. In the above motor driving assembly, since the torsional spring 79 is used to provide a radial force to the connecting base 75, the impeller can be driven to rotate only in a single direction. Therefore, the conventional motor driving assembly cannot meet the bidirectional rotation need that may be possibly desired during fabrication, assembly and use of the fan.